(Chapter 14) Footnote 6:

Galen had variously stated that the sacrum is made up of four parts, De usu partium 4.50.15, 4.66.15 (May pp. 574, 582), or of three ( De ossibus 11). As later in this chapter when he implicitly corrects Galen on the number of nerves (see note 24), Vesalius injects the phrase in homine on the assumption that Galen’s information came from the anatomy of other animals — where the number of fused sacral bones varies between three and four. Berengario da Carpi’s Isagogae breves (1522-35) shows the same inconsistency (Singer 1959, pp. 41 and 158); Niccolò Massa’s Liber introductorius (1536) said there were four, and believed they were held only by a ligament (Lind 1975, p. 251). On the Renaissance controversy that bubbled up around these bones of contention, see Sylvius 1555 pp. 86-87, chapter 18 below (pp. xxx-xxx and n. 12), and May p. 574 n. 46. The 1555 edition omits all this, stating only that the sacrum consists of six bones. Modern anatomists agree that the sacrum is a fusion of five vertebrae.